An oscillation circuit (a crystal oscillator) using a piezoelectric resonator such as a crystal resonator obtain an oscillation frequency with a relatively high degree of accuracy and so has been widely used. For example, in a radio communication system, a phase locked loop (PLL) that generates a local oscillation signal necessary for modulating a received signal and demodulating a transmission signal is mounted inside an integrated circuit (IC). The PLL circuit includes a voltage controlled oscillator (VCO). The PLL circuit generates an appropriate local oscillation signal by changing a control voltage or a phase so that an output frequency multiplied by a reference clock is obtained. However, since an IC has a manufacture variation, in each sample, the reference clock and the output frequencies of the PLL circuit slightly different from each other.
Such a variation is adjusted in a digital baseband unit through an auto frequency control (AFC) circuit that matches a frequency of a reference clock of a terminal with a frequency of a reference signal from a base station during communication. The AFC circuit has a controllable range, and an allowable frequency variation of the reference clock is limited to the controllable range. Thus, the reference clock generally requires a high degree of accuracy of the order of tens of parts per million (ppm) with respect to the voltage, the temperature, and a manufacture variation. For this reason, there has been used a voltage controlled temperature compensated crystal oscillator (VCTCXO), which is an oscillator having a small frequency variation, an expensive external component.
In recent years, in a radio communication system, the miniaturization, lightening, and cost reduction has been strongly demanded. Particularly, since the demand for cost reduction is high, an oscillation circuit employing a piezoelectric resonator (a crystal resonator) such as a quartz crystal is attracting attention as an inexpensive oscillation circuit that substitutes for the VCTCXO.
The oscillation circuit has a simple configuration in which an inexpensive piezoelectric element such as a quartz crystal, an IC internal oscillator such as a complementary metal oxide semiconductor (CMOS) inverter, and variable IC internal load capacitors connected to an input and an output of the CMOS inverter are mounted. However, a variation of the piezoelectric resonator such as a quartz crystal is large, and a degree of frequency accuracy is not as high as the VCTCXO. The frequency of the oscillation circuit is decided depending on the piezoelectric resonator such as a quartz crystal and a load capacitance value inside the IC. Thus, the frequency variation is reduced by minutely adjusting the variable capacitance value inside the IC.
An oscillation circuit is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 7-131247, 11-330856, and 2006-157767.
However, the crystal oscillation circuit has a problem in that since there are not only a variation of the piezoelectric resonator such as a quartz crystal influenced by the temperature, but also an internal load capacitance variation in the IC, the frequency of the reference clock that is not adjusted yet is greatly different from a desired value and deviates from the controllable range of AFC control inside the communication system. The load capacitance variation is reduced by mounting a high-accuracy IC external component. However, in this case, the number of components increases, and miniaturization and cost reduction are difficult to achieve. Further, in order to directly detect that an oscillation frequency has been deviated, a high-accuracy frequency detector of the order of ppm is necessary. However, this technique is not suitable for miniaturization and cost reduction.
As described above, the oscillation circuit employing the piezoelectric resonator such as a quartz crystal is required to simply reduce the oscillation frequency variation at a low cost.